In accordance with explosive increase in demand for broadband multimedia communication services such as the internet and video delivery, introduction of a high-density wavelength-division multiplexing fiber optic communication system, which has features of longer distance, larger capacity and higher reliability, has been set forward in core and metro networks. Also in subscriber access networks, an optical fiber access service has become in widespread use rapidly. In such communication systems using optical fibers, it is important to reduce the cost for laying optical fibers to be optical transmission lines and to increase the transmission band utilization efficiency for each optical fiber. For this reason, widely used is a wavelength-division multiplexing technology which transmits a plurality of optical signals of different wavelengths by multiplexing them with each other.
In an optical transmitter for the use in a wavelength-division multiplexing fiber optic communication system, there is requirement for an optical modulator which is capable of high-speed optical modulation with small dependence on optical signal wavelength, where an unnecessary optical phase modulated component (when an optical intensity modulation method is used as the modulation method) or an unnecessary optical intensity modulated component (when an optical phase modulation method is used as the modulation method), both causing degradation in the waveform of received light in a case of long-haul signal transmission, is suppressed to the utmost. For such a use, usually used is a Mach-Zehnder (MZ, hereafter) optical intensity modulator incorporating a waveguide type optical phase modulator, which is similar to a waveguide type MZ interferometer.
For increasing the transmission capacity per wavelength channel, from the viewpoints of spectrum utilization efficiency and of tolerance to wavelength dispersion and polarization mode dispersion of an optical fiber, a multi-level optical modulation signal method, which has a narrower optical modulation spectrum bandwidth compared to a usual binary optical intensity modulation method, is advantageous. Further, in a system where longer-haul transmission is required, such as that used for trans-oceanic optical communication, there also arises necessity for a pre-equalization technology which compensates a waveform distortion generated in an optical fiber being the transmission line by applying a distortion of the inverse characteristic at the sending side. To enable application of such multi-level modulation and a pre-equalization technology, the sending side is required to have a function of digital-to-analog converter (DAC) which is capable of generating an optional signal waveform produced by digital signal processing. For example, Patent Document 1 discloses a method which drives an optical modulator of the MZ configuration, where an LN modulator or the like is used, with analog signals using an electrical DAC and a linear amplifier.
To increase the transmission capacity per wavelength channel, improvement in the symbol frequency of data is also needed. For improvement in the symbol frequency of data, the band of a device is an important factor. Taking as an example an optical device generally used in fiber optic communication systems (for example, an optical modulator), its band is restricted by restriction of CR time constant due to mainly the influence of a resistance component R and the capacitance C of the device. Because such an optical device uses interaction between light and an electric field, a required voltage and a capacitance of the device is determined by the electric field strength and the interaction length. It is general that, with increasing the interaction length, the electric field strength per unit length is allowed to be smaller, but the capacitance of the device increases at the same time. Accordingly, in an optical modulator, for example, the power consumption and the extinction characteristic are each in a trade-off relation with the band. As a result, for such an optical device, there is no choice but to make a compromise design with consideration of a trade-off relation such as described above. In this respect, for example, Patent Document 2 proposes a method which solves the trade-off relation between the interaction length and the band. In Patent Document 2, it is proposed to reduce the capacitance by employing a split electrode structure obtained by splitting an electrode into two or more segments along the propagation direction of light and thereby driving the long electrode in a manner of driving its split segments independently in an electrically separated fashion.